Recently, friction stir welding has come to be used for joining materials (see Patent Literature 1: JP-2005-177844 A).
In the friction stir welding, a rotating welding tool is pressed onto to-be-joined objects to soften and join the to-be-joined objects with frictional heat.
In the friction stir welding, the welding tool is attached to a spindle of a friction stir welding machine and is rotated around a rotation axis intersecting the surfaces of the to-be-joined objects. Then, the welding tool is moved along a to-be-joined portion on the surfaces of the to-be-joined objects while being rotated.
The friction stir welding machine is often a dedicated device for applying pressure on the objects (i.e. a workpiece) to generate the frictional heat by an axial movement (movement of the tool and/or the workpiece) while rotating the welding tool with a high torque. On the other hand, general-purpose machine tools can replace the friction stir welding machine as far as being used for rotating the spindle, and moving the tool and/or the workpiece.
The usable welding tool includes, for instance, a cylindrical tool body and a conical pin coaxially and downwardly provided on a lower end of the cylindrical tool body. A shoulder defining a step is provided around the pin.
During the progress of the friction stir welding, the pin enters the inside of the to-be-joined objects and the shoulder is kept at a position facing the surfaces of the to-be-joined objects. The friction between the rotating pin and the to-be-joined objects generates a high temperature at the friction portion, whereby the to-be-joined objects are stirred to be joined.
As described above, during the welding operation of the friction stir welding, the temperature of the to-be-joined portion becomes high and the heat is conducted through the welding tool to a spindle of a machine tool.
The dedicated friction stir welding machines have mechanism(s) against the heat from the welding tool. However, general-purpose machine tools are only designed to endure a high temperature caused by the heat conduction to the spindle or the spindle head during a typical machining.
Accordingly, when the friction stir welding is performed with a welding tool attached to a general-purpose machine tool, the spindle, a bearing supporting the spindle and the like mechanism may be adversely affected by the heat conducted to the spindle. Examples of possible adverse effects include: damages on a main bearing of the spindle head; damages on a retainer portion; center runout of the rotation axis due to thermal expansion; and fretting damages at the tapered shank.